The invention relates to a color display device comprising a color cathode ray tube including an in-line electron gun for generating three electron beams, a color selection electrode and a phosphor screen on an inner surface of a display window, and a means for deflecting the electron beams across the color selection electrode, the color display device comprising a first and a second means arranged at some distance from each other to dynamically influence the convergence of the electron beams so as to decrease the distance between the electron beams at the location of the deflection plane as a function of the deflection in at least one deflection direction, the first means being arranged in or near the pre-focusing portion of the electron gun.
Such a display device is known from international patent application no. WO99/34392.
The aim is to make the outer surface of the display window flatter, so that the image represented by the color display device is perceived by the viewer as being flat. However, an increase of the radius of curvature of the outer surface will lead to an increase of a number of problems. The radius of curvature of the inner surface of the display window and of the color selection electrode should also increase, and, as the color selection electrode becomes flatter, the strength of the color selection electrode decreases and hence the sensitivity to doming and vibrations increases. An alternative solution to this problem would be to curve the inner surface of the display window more strongly than the outer surface. By virtue thereof, a shadow mask having a relatively small radius of curvature can be used. As a result, doming and vibration problems are reduced, but, other problems occur instead. The thickness of the display window is much smaller in the center than at the edges. As a result, the weight of the display window increases and the intensity of the image decreases substantially towards the edges.
The known color display device comprises a first and a second means arranged at some distance from each other to dynamically influence the convergence of the electron beams so as to decrease the distance between the electron beams at the location of the deflection plane as a function of the deflection in at least one deflection direction. By virtue thereof, the distance between the electron beams (also referred to as the xe2x80x98pitchxe2x80x99) in the plane of deflection can be changed dynamically in such a manner that this distance decreases as the deflection increases. By dynamically changing this distance (the pitch) as a function of deflection, and hence as a function of the x and/or y coordinate(s), the distance between the display window and the color selection electrode can increase in the relevant deflection direction. The shape of the inner surface of the display window and the distance between the display window and the color selection electrode determine the shape, and in particular the curvature, of the color selection electrode.
Since the distance between the electron beams decreases as a function of the deflection, the distance between the display window and the color selection electrode increases and the shape of the color selection electrode can deviate more from the shape of the inner surface of the display window than in previous cathode ray tubes, in particular the curvature of the color selection electrode is larger. Such a larger curvature (or, in other words, a smaller radius of curvature) increases the strength of the color selection electrode and reduces doming and microphonics.
In the known color display device, the first means comprises one or more components of the pre-focusing portion of an electron gun. The outermost apertures of the G2 and G3 electrodes are offset with respect to each other and a dynamic potential difference is applied between them. In this manner, a dynamic electric field is used to influence the convergence (or divergence) of the electron beams.
Providing such a dynamic potential difference, however, entails providing a dynamic voltage difference between electrodes. This requires a separate voltage supply circuit, which is relatively expensive. Some guns use a DAF (Dynamic Astigmatism and Focus) design in which a dynamic voltage is supplied to the G3 electrode. This dynamic voltage, however, is usually mainly dependent on the horizontal deflection, rather than on the vertical deflection. The dynamic voltage range is very limited as a function of vertical deflection enabling only limited influence on the convergence.
It is an object of the invention to provide a color cathode ray tube of the type mentioned in the opening paragraph, in which the outer surface may be flat or substantially flat, while, at the same time, the above problems are overcome or reduced.
To this end, the color display device in accordance with the invention is characterized in that the first means comprises generating means for generating, outside the neck of the cathode ray tube, a dynamic magnetic field and conducting means inside the neck of the cathode ray tube and in or near the prefocusing portion of the electron gun to conduct the magnetic field to a position near the outer electron beams so as to form a local magnetic field for influencing the electron beams.
In the device in accordance with the invention, there is no need for an extra supply circuit for supplying a dynamic electric potential. In the color display device in accordance with the invention, a local magnetic field is generated for influencing the electron beams. An important advantage is also that this correcting field may be used at will, i.e. the set maker may make use of the invention when wanted, without having to change the gun or the supply circuit to the gun.
Preferably, the conducting means are attached to the G2 or G1 electrode. This enables a magnetic field to be attained very close to the cross-overs of the electron beams. Preferably, the conducting means are arranged at a surface of the G2 electrode facing the G1 electrode or at a surface of the G1 electrode facing the G2 electrode, placing the local magnetic fields even closer to the cross-over. By placing the local magnetic fields close to the cross-overs of the electron beams, a situation is attained where the beams remain converged on the screen. This is due to the fact that the main lens has the task of creating a sharp image of the cross-overs on the screen.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.